Each of these categories uses different platforms on a daily basis. But the types of games that they can play in each particular platform may differ. It's important to keep this in mind, as a really good game can be a financial failure if it was done for the wrong platform. Let's look at them in detail.
The platform where independent gaming started is the platform of biggest revenue for casual games.
But casual games downloads are going through a crisis. There is a number of good studios cranking out a far higher number of good games than the audience can consume, creating a glut of good titles and reducing the shelf life of every title.
This crisis is accentuated by the fact that portals haven't turned out to an Amazon or Netflix model; instead, they pushing the same top 10 to every user, regardless of the user's tastes.
This has forced portals to look only for titles that appeal to the vast majority of people. Genres like time management, hidden object, lightweight adventure, puzzle, and simulation are well accepted by the audience, and if you can innovate in these genres then you have a good opportunity.
Just keep in mind that in order to make your money back you need to stay a good number of weeks in the top 10 of the major portals, and that means that you need to reach at least top five in most of them to have enough momentum.
Every other genre is likely to be rejected by the mainstream portals. And it's because it's a chicken and an egg situation: with their "one top 10 for everybody formula" they have only been able to convert their original audience to paying customers: 35+ females.
It's not that males aren't playing casual games; in fact, the Casual Games Association, in their last market report, showed that 48% of casual gamers are male -- but 76% of casual gamers who pay for a downloadable game are female. Well, d'oh.
Most males I know won't be willing to pay for a product that isn't designed for them in the first place. Imagine what Wal-Mart's revenues would be if the only thing they have is stuff for your mom.
There are always exceptions for everything. Studios like PopCap, Hipsoft, Last Day of Work and Introversion, among others, have found their audiences and keep cranking innovative and interesting games out there.
And a very interesting opportunity is Steam, the home of Portal, among other good games. There are no direct links for developers to submit their wares, but if you go into Valve's web site you'll find an email address. According to this discussion at IndieGamer, Valve is listening.
Russell Carroll, of GameTunnel, was interviewed by IndieGames.com and had more to say about the subject. Then Russell, myself and other people had a discussion of the problem at IndieGamer. And some other great sites to follow what's going on in the scene are Game Tunnel and TIGSource.